I need another scratch for photoshop porformance. I was wondering whether a USB stick (USB 2.0) or another Hard Drive (IDE) would be good.

Any other options? Maybe a flash Hard Drive? Like for Compact Flash adaptor? I'm looking more for speed and performace than cappacity.

If you have a good amount of RAM and don't use many resource-intensive apps in the background (esp. P2P & Audio/Video), you may not need an extra scratch disk.

RAM

The best improvement for speed would be going to the menu Preferences... and increasing the Memory Usage setting.

65%-75% would be a good starting point. I've read some people being happy with 90%+ (a bit high for me since I often have several things running when I use PS). Also, I keep the cache level at 4.

Adding more RAM to your PC and taking advantage of PS's Memory Usage would be the best way to go IMO.

(I was going to discuss using a RAM drive as an option but it would be unnecessary because of the above setting.)

HD

If you are using huge files or have a large number of layers open at once then RAM may not be enough.

Adding a dedicated high speed HD on it's own channel would be best.

Try not to use an old HD you liberated from a PC that is older than you are. Remember, since speed is your concern, you want the fastest HD you can possibly find and ideally use it only for cache/swap. By not storing your personal files or apps on the HD you can avoid ever having to defrag the drive as well.

(Just a reminder to anyone reading this... creating a new partition on your primary HD and using it for swap space is one of the worst things you can do.)

USB, Flash....

No.

Though I don't mind discussing the reasons about why not if you really want to.